It is currently believed that certain pollution problems can be alleviated by resorting to the fluid bed combustion of coal in power generation. In this process coal particles are combusted while in violent motion in a bed fluidized by combustion air. In addition to coal, the bed is also provided with a material such as limestone in order to minimize sulfur dioxide in the exhaust gas. As a consequence, the fluidized bed contains not only coal, limestone and combustion air but also carbon monoxide, carbon dioxide, water vapor, nitrogen, fly ash, sulfur and sulfur oxides and hydrocarbon and carbonaceous products of combustion and pyrolysis. The fluidized bed also contains quasi-molten solids comprising mixtures of fly ash and calcium sulfate which can adhere to and coat structures in contact with the bed. Although combustion air is generally supplied in about 20% excess of that required to combust the coal, it is not unusual to measure PO.sub.2 of 3.times.10.sup.-12 to 10.sup.-16 and even to 10.sup.-22 in certain parts of the fluid bed rather than the PO.sub.2 of 3.times.10.sup.-2 suggested by the excess air present.
The fluid bed combustion of coal is generally designed to be carried out in a reactor where the containing walls comprise tubes carrying liquid water and/or steam which liquid water and/or steam is a medium for recovering thermal energy from the reactor. Regions of temporary or quasi-permanent low partial pressure of oxygen in the fluid bed coal combustor are most likely to be found adjacent the "water wall" tubes and in areas adjacent the coal feed mechanisms to the fluid bed. It is in these regions that water wall tubes and other structures can fail due to carburization, sulfidation or combinations of either or both of these mechanisms along with oxidation.
Those skilled in the art will appreciate that water-wall tubes in a fluidized bed coal combustion are boiler tubes and as such are governed by the requirements of the boiler code.
Boiler tubes must possess a combination of mechanical and corrosion resistant properties. Material requirements are defined in ASME materials specifications SA213, SB163, SA540 and ASME Code Case 1874. Corrosion resistant properties are equally as exacting if not more difficult to define. However, it is clear that the material of construction of choice for fluid-bed combustor tubing must possess corrosion resistance to sulfur-containing atmospheres under a wide range of oxygen partial pressures. Current materials of construction that are otherwise in compliance with the ASME specifications lack corrosion resistance to these atmospheres. This is true because most of these alloys develop chromia scales for corrosion protection. Below partial pressures of PO.sub.2 of about 10.sup.-20 atm, chromia becomes unstable in carburizing atmospheres and below about 10.sup.-22 atm. in sulfidizing atmospheres depending upon the temperature and the actual partial pressure of the mixed oxident. In alloy scale where chromia is diluted with nickel oxide, iron oxide and other oxides the effective PO.sub.2 at which the chromia in the scale converts to chromium carbide can be as high as 10.sup.-10 or, more likely as high as 10.sup.-12 to 10.sup.-16 atm. Materials that possess corrosion resistance to these atmospheres either do not have the required mechanical properties or other code requirements for boiler tube applications. One solution is a composite tube with an inner core of an alloy meeting the boiler tube specifications for mechanical properties and an alloy for the outer layer which possesses corrosion resistance in the mixed oxidant environment of the coal fired, fluidized-bed combustoer. Such an alloy must be highly compatible with the substrate alloy, readily fabricated and welded and must be resistant to thermal instability and the formation of embrittling phases at the interface between the two alloys.
It is the object of the present invention to provide such an alloy and to provide composite tubes, the outer layer of which consists of or comprises such alloy.